As being self-luminescent, organic electroluminescent, EL devices have high visibility. In addition, they have high impact resistance as being completely solid devices. Therefore, they are much used in various fields of thin-film display devices, back lights for liquid-crystal displays, flat light sources, etc.
Distributed electroluminescent devices are now in practical use. As they require alternating voltage of at least tens volts and 10 kHz or more, their driving circuits are complicated.
In the circumstances, organic EL devices capable of being driven at lowered voltage of 10 volts or so and capable of emitting high-luminance light are much studied these days. For example, thin-film organic EL devices having a multi-layered structure of transparent electrode/hole injection layer/emitting layer/back electrode are proposed in Appl. Phys. Lett., Vol. 51, pp. 913–915 (1987) by C. W. Tang and S. A. Van Slyke, and in Japanese Patent Laid-Open No. 264629/1988. These are so designed that the hole injection layer therein can efficiently inject holes into the emitting layer therein. The emitting layer in such organic EL devices may have a single-layered structure, which, however, could not enjoy well-balanced electron transportation and hole transportation. To solve the problem, the emitting layer is modified to have a multi-layered structure of improved performance.
However, the process of forming the multi-layered emitting layer is complicated and takes a lot of time. Another problem with it is that the multi-layered structure is against the recent tendency in the art which is toward reducing the thickness of layers constituting organic EL devices. On the other hand, down-sized, compact and portable information appliances are much desired these days, and they are required to be driven at low voltage. In the circumstances, various types of light-emitting materials and hole-transporting materials are tried for such lightweight, low-voltage driving appliances.
Further, the most important theme in practical studies of organic EL devices is to establish the technique of preventing the attenuation of the luminance of the devices in long-term driving and to provide practicable organic EL devices. In this respect, it is said that the purity of organic compounds to be used for producing constituent materials for organic EL devices has a great influence on the attenuation of the luminous efficiency and the luminance of the devices produced, for example, as in “Monthly Display, September, p. 15, 1995”, and “Applied Physics, Vol. 66, No. 2, pp. 114–115, 1997”. However, the influences of the structures and the properties of organic compounds to be used for producing organic EL devices on the properties of the devices produced are not as yet clarified, and no method has heretofore been established capable of quantitatively determining the influences in question.
In that situation, the object of the present invention is to provide an organic EL device having the advantages of applicability to lightweight, thin and low-voltage driving displays, good luminescent capacity attenuating little even in long-term driving operation, and good durability.